Method and apparatus for stacker module for automated composite-based additive manufacturing machine

ABSTRACT

A stacker component of an apparatus for automated manufacturing of three-dimensional composite-based objects for aligning registration of sheets. The stacker includes a sheet catcher; a frame having a base plate with the base plate having tapered registration pins to align a stack of substrate sheets. The registration pins are mounted in the base plate and project vertically to a location just below the sheet catcher. The stacker also has a presser with a press plate and a belt driver system that moves the press plate up and down allowing the press plate to exert downward pressure on the stack and a slide system with two guide rails that enable the base plate to be loaded and unloaded. A conveyor can be disposed so that after a substrate sheet exits a powder or printing system, the sheet is conveyed onto the sheet catcher.

This application claims the benefit of U.S. Provisional Application No.62/473,062, filed Mar. 17, 2017. Application 62/473,062 is herebyincorporated by reference in its entirety.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to additive manufacturing and, inparticular to an apparatus that is a stacker component of an apparatusfor automated manufacturing of three-dimensional composite-basedobjects.

BACKGROUND OF THE INVENTION

Additive manufacturing, such as three-dimensional printing, can be seenas largely a materials problem. One of the limitations of currentmethods is a limited materials palette and slow build speeds.

These and other limitations of the prior art are avoided by amethodology known as Composite-Based Additive Manufacturing (CBAM). CBAMis described in full in co-pending U.S. patent application Ser. No.13/582,939, filed Nov. 2, 2012, Ser. No. 14/835,690, filed Aug. 25,2015, and Ser. No. 14/835,635, filed Aug. 25, 2015, each of which isincorporated fully herein by reference.

International application no. PCT/US17/17672, filed Feb. 13, 2017,describes a particular method and apparatus for automatingComposite-Based Additive Manufacturing (CBAM). International applicationno. PCT/US17/1772 is incorporated fully herein by reference.

A problem arises during the automated design and manufacture of a 3Dobject, using the CBAM method, which is how to get sheets automaticallystacked on top of each other while holding precise sheet-to-sheetregistration.

Use of registration pins as a way of registering things together is amethod that is widely used. For example in metal stamping registrationpins can be used in stages to assure that multiple steps will be done inregister. A similar process is used in making negatives for four colorprocess printing.

SUMMARY OF THE INVENTION

This application describes a particular stacker method and apparatuswhich is a component of an overall machine for automatingComposite-Based Additive Manufacturing (CBAM). The stacker may be an endcomponent of the machine, enabling the stacking of sheets on top of eachother with a high degree of sheet-to-sheet registration. With the designof the present invention, the overall precision of the machine can berelaxed except for a few key components.

As substrate sheets go through the CBAM machine, respective layers of a3D object (“layer images”) are printed on respective substrate sheets.To form the 3D object from the layers, each substrate sheet needs to beappropriately aligned with the sheet above and below it so that therespective layer images printed on those sheets are aligned in register.The stacker helps to do this by causing the substrate sheets to stack ontop of each other in register. This is accomplished by providingregistration holes on each sheet in a predefined relationship to theposition of the layer images printed on the sheet, so that each layerimage properly aligns with the one above and below it when the sheet ispositioned on the registration pins that run through the stacker. If theregistration holes are not properly oriented in relationship to theposition of the layer image on the substrate sheet, then the part formedfrom the layers will have poor tolerances or the part may even fail toform.

The relationship between the registration holes and the image orientedon the sheet is critical for precise sheet to sheet registration. Thusif the relationship between the registration holes and the image areprecisely fixed, it is possible to achieve precise registration betweenthe sheets. If the sheet is punched with an apparatus that is fixed inits relationship to the printer, this relationship can be repeatable andprecise. Using this approach one can build a device where precision islargely limited to this relationship.

One approach to this problem is to have the sheets stack on taperedpins. The end of the pin can have a diameter of ⅛″, while the hole inthe sheet can have a diameter of ¼″. Thus if the there is somemis-registration of the punch and the corner of the sheet, the sheetwill still fall on the pin.

Thus, registration does not need to be exact when the sheet is fed bythe material feeder, and placement of the substrate sheet on the printerplaten does not need to be exact when the sheet is printed and punched.Instead, accurate sheet-to-sheet registration results from there beingvery good registration between the location where the sheet is punchedand the location where it is printed upon. For example, precisionbetween punching and printing locations may be a few thousandths of aninch, while precision of placing the sheet onto the printer platen couldbe as much as an eighth or quarter of an inch. The metal frame of thestacker ensures that registration achieved upon printing and punching ispreserved within a few thousandths of an inch.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of an example embodiment of an apparatus forcomposite-based additive manufacturing.

FIG. 2 depicts a roll-based continuous feed apparatus forcomposite-based additive manufacturing.

FIG. 3 depicts a roll-based roll to sheet apparatus for composite-basedadditive manufacturing.

FIG. 4A illustrates a top view of a registration pin with a taper.

FIG. 4B illustrates a side view of a registration pin with a taper.

FIG. 5 illustrates a printed image on a substrate sheet in a positionrelative to punched registration holes.

FIG. 6 shows another embodiment of a stacker.

FIG. 7 is another view of an example stacker (excluding the bottom).

FIG. 8 is a schematic of a stacker.

FIG. 9 is a schematic side view of the stacker.

FIG. 10 is another schematic side view of the stacker.

FIG. 11 is a view of the stacker with its base plate loaded.

FIG. 12 is another view of the stacker with its base plate unloaded.

FIG. 13 is a view of a conveyor.

FIG. 14 is another view of the conveyor.

FIG. 15 is another view of the conveyor.

FIG. 16 is another view of the conveyor.

FIG. 17 is another view of the conveyor.

DETAILED DESCRIPTION OF INVENTION

Overall Machine

The CBAM process described in the incorporated prior applications (U.S.patent application Ser. Nos. 13/582,939, 14/835,690, and 14/835,635) isautomated by performing the steps through a number of components orsubsystems that operate in a coordinated manner. A machine thatautomates the steps is described in international application no.PCT/US17/17672 and U.S. application Ser. No. 15/611,320. The maincomponents of an example embodiment of the machine 100 are shown in FIG.1, and include a material feeder 102, a printer 104, a powder system 500comprising a powder applicator 530 and powder remover/recycler 532, anoptional fuser 112, a transfer system, and other elements that serve toconnect and control the various components. While example components areshown in FIG. 1, various alternative and optional components are alsosuitable for use with the machine 100.

The material feeder 102 holds a stack of substrate sheets 101, such ascarbon fiber sheets, and moves them into proper position so that asingle sheet 101 at a time can be transferred to the printer platen 300and printer 104. Sheets 101 are transferred to, and positioned for, theprinter 104 by means of the transfer system. The printer 104 thendeposits fluid onto a substrate sheet 101 as described in theincorporated prior applications (U.S. patent application Ser. Nos.13/582,939, 14/835,690, and 14/835,635), and includes a punchingmechanism for placing registration holes in the sheet 101 at desiredlocations. The registration holes are placed in precise, pre-definedpositions relative to the position of the layer images printed onto thesheets. This can be accomplished by mounting the punches on the sameframe on which the printing mechanism is placed. The powder applicator530 then deposits thermoplastic powder onto the substrate sheet 101,whereupon the powder adheres to the areas of the sheet 101 that havebeen made wet by the printer 104, i.e., the layer images. The powderremover/recycler 532 removes any powder that did not adhere to the sheet101. The fuser 112, which is optional, heats the powder on the substratesheet 101 in a manner sufficient to cause the powder to melt and therebyaffix to the sheet 101, so that the powder remains on the sheet 101 whenand if the underlying fluid from the printer 104 dries. The apparatusalso includes a stacker subsystem 400 for stacking the sheets inregister. This cycle is repeated for as many additional substrate sheets101 as required for making a specified three-dimensional (3D) part, witheach sheet 101 normally representing a layer of the 3D part.

Also shown in the embodiment of the machine depicted in FIG. 1 is adistance sensor 138, Coanda or felted-material gripper 118, XYZpositioner 116, X positioner 126, Y positioner 128, print heads 105,needle or felted-material gripper 120, rails 114, conveyor 152, cyclone154, and air knife 160. These components are described in detail ininternational application no. PCT/US17/1772 and U.S. application Ser.No. 15/611,320.

Instead of using substrate sheets, a roll of substrate material may beused in the CBAM process and automated machine. FIG. 2 depicts acontinuous feed roll implementation 190, and FIG. 3 depicts a roll tosheet implementation 195. In these embodiments, a roll of substratematerial 102 is mounted and situated ahead of the printer 104. Atensioning system 103 together with feed rollers 106 are used to holdand advance the web defined by the length of the roll material fedthrough the system. The web 102 can extend through all of the componentsof the system—printer 104, recycler 500 comprising powder applicator 530and powder remover/recycler 532, and, if present, fuser 112—and then becut by a cutter 130 into single sheets 101 prior to stacking by thestacker subsystem 400. This is depicted in FIG. 2. Alternatively, asdepicted in FIG. 3 the web 102 may be cut by the cutter 130 into singlesheets 101 at any prior point in the process. For example, the web 102may be converted to a single sheet 101 prior to advancing the resultingsheet 101 onto the printer platen 300. The web 102 may be converted to asingle sheet after the leading edge is situated on the platen 300. Theweb 102 may be converted to a single sheet after the printing operationis completed and before the resulting sheet is fed into the powderapplicator 530, and so on.

Motivation for Having a Stacker Subsystem

The CBAM process prints each layer of a 3D object on a respectivesubstrate sheet. During the printing operation, the CBAM process alsopunches registration holes in the substrate sheets 101. The holes arepunched in registration to where the object layers are printed on thesheet, so that when the sheets are stacked on pins 414 in register ontop of each other, the printed object layers are also in register. Theselayers are then fused together, and the excess material is removedproducing a 3D object in the shape of the original 3D model. In orderfor the 3D object produced to accurately reproduce the model, theregistration of the location of the holes are punched in the substratesheets in relation to the location where the object layers are printedon the sheet must be accurate. Any inaccuracy in the registrationbetween the holes and the printed areas will cause inaccuracy in thesheet-to-sheet registration and will be reflected in the 3D object thatis produced. In particular, vertical edges and surface finish are verysensitive to mis-registration of sheets, as are feature definition anddimensional tolerances.

Because the present invention uses registration pins 414 to align theprinted sheets, the locations of where the layer of the object isprinted on the sheet must be accurately correlated with the pins 414.That is, the relationship of the printed layer to the registration pins414 must remain the same for each sheet that is printed. This is a morestringent requirement than typical printing, because in typical printingthere is no requirement for sheet-to-sheet registration. Hence, this isnot something that is typically done in conventional printing. In thepresent invention, registration holes are punched with the samemechanism that performs the printing so that the relationship betweenthe holes and the printed image remains the same. This can also be doneby other methods so long as this relationship remains unchanged for eachsheet printed. It is important to note that using this methodology forprecision allows other parts of the machine system to be less precise.This has a number of advantages, including making the other parts easierto design, less expensive to make, and the overall machine easier tomanufacture and calibrate.

In the present invention, the substrate sheet 101 is typically picked upby a gripper 118 or other method as previously described, then placed ona platen 300. The level of registration that is required for placing thesheet 101 on the platen 300 is around ⅛ to ¼ inch. Once the substratesheet 101 is on the platen 300, it is printed on, and registration holesare punched with (a) the printing being done in precise relationship tothe location on the sheet where the holes are punched, and (b) thealignment between the punch or punches being precise and repeatablebetween sheets 101. Alternatively, this punching can be done at otherpoints in the process as long as precision between registration holelocations and printing locations are maintained.

After printing and punching, the sheet 101 is typically picked up by aneedle gripper or felted-material gripper 120 and placed on a conveyor152. It is then powdered and vacuumed so that powder remains on thesheet 101 where the image was printed. This makes up one layer of the 3Dobject. Again, this step requires no precise registration. After this,the sheet 101 is transferred to another conveyor 152 (or it could remainon the same conveyor) where it is aligned by channels 153 on either sideof the conveyor 152 (note FIGS. 13-17). The channels 153 align the sheet101 if it is not rectilinear with respect to the conveyor 152. Then withboth air and the momentum of the sheet 101 from the conveyor 152, thesheet 101 goes into a stacker 400. In the stacker 400 there are rightangle lengths of sheet metal that act as corners and align the backcorner of the sheet 101. The sheet is held up by spring steel 418 andthen pressed over registration pins 414 onto the stack.

The punched holes in the sheet 101 are slightly larger than the diameterof the main body of the registration pins 414. The difference indiameter between the punched holes and the registration pins 414 shouldbe enough so that the sheets can be stacked tightly onto the pins 414without puckering, tearing or otherwise deforming. The registration pins414 can be tapered 416 at their top ends as shown in FIGS. 7-10. Theends of the pins can be around ⅛ inch in diameter whereas the punchedholes can be larger, such as ¼ inch. This allows there to be someinaccuracy in the initial placement of sheet 101 on the stacker as itrelates to the pins 414 and still allows the sheet 101 to be placed ontothe pins 414 (at the tapered ends 416) but then achieve preciseregistration as the sheet travels down onto the main body of the pins414. The difference between the top of the tapered pin 416 and thebottom can vary based on various criteria, among them the desiredprecision of the system.

Another element of this system is that the plate 412, which holds thestack of material, can be mounted on rods or guide rails 420 (as shownin FIGS. 11-12) and slid backward out of the main body of the stackermechanism so that the stack of material can be removed from the plate,or removed together with the plate, so that it can be placed in a jigthat is then compressed and heated as part of the CBAM process.

FIG. 4A, FIG. 4B and FIG. 5 help to further illustrate the stacker 400.FIG. 4A illustrates a top view of a registration pin 414 with a taper416. FIG. 4B illustrates a side view of a registration pin 414 with ataper 416. FIG. 5 illustrates an arbitrary printed layer image 432 on asubstrate sheet 101 in a position relative to punched registration holes430.

The diameter of the registration pin 414 below the taper 416 is ofsimilar size to the diameter of the registration holes in the substratesheets, and preferably just slightly smaller than the registration holesin the substrate sheets. The diameter of the registration pins along thetaper area 416 decreases substantially. This allows the substrate sheetto be easily placed on the registration pins, because the ends of thepin tapers 416 are much smaller than the diameter of the registrationholes. Then, when the substrate sheets travel down the pins 414 as theyare stacked, they align tightly with the pins 414 thus ensuringappropriate registration between one sheet and another.

More specifically, if the tapered pin 416/414 has a radius of r 434(diameter is 2r), then when the sheet is initially placed into thestacker with its corners abutting stops 422 (see FIGS. 6 and 7), thepunched holes 430 need only be anywhere within r 434 away from thelocation of tips 416 of the pins. Even with this imprecision, thisallows the sheet 101 to be pushed onto the pins 414 and then achievehigh precision in sheet-to-sheet registration.

The stacker 400 simplifies the design of the machine 100/190/195.Without the stacker 400, every sheet 100 would need to be at the exactsame place at every stage of the process every time (e.g., materialfeeder 102, platen 400 placement). Every stage of the process has thepossibility for error (and the error multiplies at every stage).

Stacker Subsystem

A different embodiment of the stacker 400 is shown in FIGS. 6-12. Afterthe sheet 101 has had powder applied and excess powder removed (and,optionally, the remaining powder has been fused to the sheet), thesheets are stacked in order onto registration pins to align the sheetsfor later processing to form the 3D parts. The stacking is automaticusing a stacker subsystem 400 such as shown in detail in FIGS. 6-12. Thestacker comprises a frame 402, a sheet catcher 404, and a presser 406comprising a press plate 408 and a belt driver system 410 that moves thepress plate up and down. The frame further comprises a base plate 412.Registration pins 414 are mounted in the base plate 412 and projectupwardly to a location just below the sheet catcher 404. The diameter ofthe registration pins are just slightly smaller than the size of holesthat are punched in the substrate sheets, so that any lateral movementof the sheets is minimized once the sheets are stacked onto theregistration pins. Further, the registration pins have tapers 416 attheir top ends to facilitate placement of substrate sheets onto thepins. As shown in FIG. 11 and FIG. 12, the stacker also comprises aslide system to be able to load and unload the base plate 412, pins 414,and optionally stacked sheets 101. The slide system includes two rods orguide rails 420 enabling the base plate 412 to be loaded and unloaded.The stacker subsystem 400 also includes a mechanism to insert and removepins 414 (the pins need to be aligned very accurately), as well as acomputer 424 having a processor to control the sequence of movements ofthe stacker 400.

The sheet catcher 404 typically has two leaf springs 418, and a optionalsensor (not shown) that can detect when a substrate sheet 101 is in thesheet catcher 404. The sensor can be any type of sensor, including anoptical sensor, suitable for determining the presence or absence of anitem. The conveyor 152 of the powder system 500 is disposed so thatafter a substrate sheet 101 exits the powder system 500, it is conveyedonto the sheet catcher 404 of the stacker system 400. The sheet catcher404 includes stops 422 so that as the sheet 101 releases from the end ofthe conveyor into the sheet catcher 404, the stops 422 cause the sheetto stop its motion of travel and come to rest on top of the leaf springs418. Then, the sensor, which detects the presence of the sheet, sends asignal causing the belt driver 410 to move the press plate 408 downward.As the press plate 408 moves downward, it depresses the leaf springs 418thereby allowing the substrate sheet 101 to be pressed onto anddownwardly along the registration pins 414. The press plate 408continues downward until it reaches the base plate 412 or, if substratesheets 101 have previously been stacked onto the base plate 412, the topof the stack of sheets 101. The press plate 408 then returns to itsoriginal position at the top of the stacker 400 and waits for the nextsubstrate sheet 101 to enter the stacker 400 whereupon the process isrepeated until the final substrate sheet 101 needed to make the 3D partis stacked. Processes downstream of the stacker 400 can be either manual(a worker carries the stack to the compression stage, then fusing stage,then abrasion stage) or automatic (not shown).

There is a tight fit between the top of the registration pins 414 (thetapers 416) and press plate 408, to minimize error. The pins 414 areinserted into collars of the base plate 412, which allow the pins 414 tobe inserted into and taken out of the base plate 412 rapidly. Thecollars also make the registration pins 414 very straight vertically forproper alignment. Because the base plate 412 is on slides (rods or guiderails 420), the stack of sheets can be taken out with the base plate 412and pins 414, and then another base plate 412 and pins 414 can beinserted into the frame 402 so that more parts can be made.

As the stack of sheets gets larger and larger, instead of having to keeptrack of how large the stack is and the location of the press plate 408,the belt 410 is connected to a presser 406 (which is the part that movesup and down), which can be off to the side of the stack and can run thefull length of motion regardless of how many sheets 101 are on it. Thepress plate 408 rests on top of the presser 406, but the press plate 408is not physically attached to the presser 406. As the presser 406descends, the press plate 408 by gravity moves along with it. The pressplate 408 then stops at the top of the stack and the presser 406continues down to the bottom of the stacker 400. When the presser 406returns upward again, it catches up with the press plate 408 and liftsthe press plate 408 up again. The presser 406 runs along the full lengthof the stacker 400. Because the presser 406 is disconnected from thepress plate 408, software does not need to keep track of how high thestack is.

Since the image is powdered, the images can be contaminated with powderfrom other images. Therefore, the press plate 406 is not just a flatplate; rather it has the shape of a cutout rectangle with cutouts wherethe pins 418 are located. The reason for the cutout rectangle is so thatit can push on unprinted margins on the sheet 101 without touching thepowder so no powder gets onto the press plate 408 or then transferredback onto another sheet 101.

The registration pins 414 typically fit in a collar so that they can betaken out with the resulting stack and transported. New pins can thenput in, and the base plate slid back via the rods/guide rails 420.Making sure that the registration pins 414 are straight is veryimportant, so there is a mechanism to keep the registration pins 414straight. A very small error at the bottom of the pins translates to thetips 416 of the pins 414 being dislocated from their intended positionsmaking it difficult to stack the sheets 101 onto the pins.

The leaf springs 418 (made of flexible spring steel) hold a single sheetin place before it is moved onto the pins 414. The leaf springs 418allow the press plate 408 to travel through. As the press plate 408presses a sheet 101 downwards, the press plate 408 deforms the leafsprings 418, thereby permitting both the plate and the sheet to passthrough. After the plate passes through, the leaf springs contract backup (and the same action is repeated, but in the opposite direction, whenthe plate travels back up to the top of the stacker).

The stacker can accommodate variations in both sheet thickness andplanar extent. In this embodiment there are several different sheetsizes. They are all typically of twelve inch width, but they can havedifferent lengths (e.g., 4 inches, 8 inches, 12 inches, and 16 inches).By changing the spring steel hinge 418 (leaf springs), or the settingson the spring steel hinge 418, the stacker 400 can accommodate differentsheet sizes, and in addition accommodate different locations in thevarious sheets sizes where the holes are punched and where the pins 418are set within the stacker frame 402).

FIGS. 13-17 illustrate an example conveyor 152 with channels 153.Conveyors are used to move substrate sheets in the system. A conveyorcan be used to move printed sheets from a platen where they are printedto a stacker represented by the present invention. The channels 153align the sheet 101 if it is not rectilinear with respect to theconveyor 152.

While the above specification and examples provide a description of theinvention, many embodiments of the invention can be made withoutdeparting from the spirit and scope of the invention. It is to beunderstood that the foregoing embodiments are provided as illustrativeonly, and do not limit or define the scope of the invention. Variousother embodiments are also within the scope of the claims.

What is claimed is:
 1. A stacker apparatus for stacking and aligningsubstrate sheets, comprising: a frame having a base plate; a sheetcatcher; a presser having a press plate and a system that moves thepress plate up and down; and a plurality of registration pins thatproject upwardly to a location just below the sheet catcher, theregistration pins having tapers at their upper ends.
 2. The apparatus ofclaim 1, further comprising a mechanism for inserting and removing theregistration pins.
 3. The apparatus of claim 1, wherein the diameter ofthe registration pins is slightly larger than the size of holes that arepunched in the substrate sheets, the holes being placed in positionsrelative to a position of layer images printed onto the sheets.
 4. Theapparatus of claim 1, further comprising a computer having a processorto control a sequence of movements of the stacking apparatus.
 5. Theapparatus of claim 1, wherein there is a tight fit between the tapers ofthe registration pins and the press plate, to minimize error.
 6. Theapparatus of claim 1, wherein the registration pins are inserted intocollars of the base plate.
 7. The apparatus of claim 6, wherein: thecollars allow the registration pins to be inserted into and taken out ofthe base plate rapidly; and the collars hold the registration pins verystraight vertically for proper alignment.
 8. The apparatus of claim 1,wherein there is no requirement to monitor the size of a stack ofsubstrate sheets in the stacker because the presser is disconnected fromthe press plate.
 9. The apparatus of claim 1, wherein the press plate islocated adjacent a stack of substrate sheets and can run the full lengthof motion of the stacking apparatus regardless of how many substratesheets are on the stacking apparatus.
 10. The apparatus of claim 1,wherein the press plate rests on top of the presser, while not beingattached to the presser.
 11. The apparatus of claim 1, wherein the pressplate is a cutout rectangle with cutouts for accommodating theregistration pins.
 12. The apparatus of claim 1, wherein theregistration pins are as tall as the entire stacking apparatus.
 13. Theapparatus of claim 1, wherein the stacking apparatus can accommodatesheets of various thicknesses and planar extent by changing leaf springsor settings of the leaf springs, locations where holes are punched inthe substrate sheets, and locations of where the registration pins areset within the frame.
 14. A method for stacking and aligning printedsubstrate sheets that correspond to cross-sections of athree-dimensional (3D) object, each sheet having a plurality of punchedregistration holes, by moving each sheet under automated control of astacking apparatus, comprising: (1) conveying the substrate sheet onto asheet catcher; (2) causing the substrate sheet to come to rest in thesheet catcher; (43) causing a belt driver to move a press plate downward4 pressing the substrate sheet downwardly such that a plurality ofregistration pins pass through holes in the substrate sheets (5)returning the press plate to an original position at the top of thestacking apparatus; and (6) repeating steps (1) through (5) until afinal substrate sheet needed to make the 3D object is stacked andaligned.
 15. The method of claim 14, wherein the stacking apparatus isthe apparatus described by claim
 1. 16. The method of claim 14, whereinprocesses downstream include a compression stage, a fusing stage, and anabrasion substrate removal stage.
 17. The method of claim 16, whereinthe downstream processes are either manual or automatic.
 18. The methodof claim 14, further comprising: removing a stack of substrate sheets,base plate, and the plurality of registration pins from the stackingapparatus via a slide system having guide rails; placing the removedstack into a jig placing the jig into an oven to produce a 3D object;inserting into a frame of the stacking apparatus a base plate and pinsvia the slide system so that another 3D object can be made.
 19. Themethod of claim 18, wherein the jig comprises: a thicker base plate thanthe base plate of the stacking apparatus; a top plate; and screwingmechanisms that put the stack of substrate sheets under pressure. 20.The method of claim 14, wherein it is not required to monitor the sizeof a stack of substrate sheets because the presser is disconnected fromthe press plate.
 21. The method of claim 20 wherein the press platerides on top of the presser such that as the presser travels downward,the press plate separates from the presser and rests on top of a stackof printed substrates.
 22. The method of claim 21, wherein when thepresser travels upward, the presser re-engages the press plate and liftsthe press plate away from the stack of printed substrates.
 23. Themethod of claim 14, wherein the conveyor and a plurality of chutes alignand constrain the sheets when conveyed to the stacker, such that thesheets align properly in the stacking apparatus.
 24. The method of claim14, wherein the conveyor has channels to align the sheets as they areconveyed to the stacking apparatus if the sheet is not rectilinear withrespect to the conveyor.
 25. The method of claim 14, wherein thedifference in diameter between the punched holes and the registrationpins is sufficient so that the sheets can be stacked tightly onto theregistration pins.
 26. A method for stacking and aligning printedsubstrate sheets that correspond to cross-sections of athree-dimensional (3D) object, comprising: punching registration holesonto the substrate sheet, the holes placed in positions relative to aposition of the layer image printed onto the sheet; and stacking andaligning the substrate sheets by pressing the substrate sheets onto anddownwardly along a plurality of registration pins via the punched holesin the substrate sheets.
 27. The method of claim 26 wherein theregistration pins are tapered with the tops of the pins having a smallerdiameter than the bottoms of the pins.